Study on the high-temperature and aging properties of agricultural waste-modified asphalt based on rheology

The modifier of road materials from agricultural waste (AW) as raw material has been widely noticed. Considering the environmental impact of AW treatment and the National policy on the promotion of resource reuse, the feasibility of four AW (namely, bamboo powder, rape straw, corn cob, and wheat straw) for styrene butadiene styrene (SBS) asphalt modification is studied from the properties and mechanism perspectives. Through properties evaluation tests (such as the dynamic shear rheometer, multiple stress creep recovery, and rotating thin film oven test), the influence of four AW and different mixing amounts on the properties of SBS modified asphalt pavement is analyzed from the aspects of high-temperature deformation resistance and anti-aging properties. The results reveal that the four AW can improve the SBS asphalt’s high-temperature deformation resistance and anti-aging properties, among which rape straw has the most significant improvement effect. In addition, through the fourier transform infrared spectroscopy test, the microscopic mechanism of the AW/SBS composite modified asphalt binder is revealed from the functional groups. The analysis shows that the AW is physically mixed with the SBS asphalt binder, which inhibits the growth of sulfoxide groups and the cracking of the SBS modifier during aging.


Introduction
Asphalt pavement has become a primary, widely used form in high-grade pavement construction owing to its good road performance and ease of construction [1]. However, cracks and other damages seriously affect its service status [2,3]. The mass proportion of asphalt in asphalt concrete is not large, nonetheless, it has a crucial impact on performance. The commonly used method for enhancing asphalt performance is modifying it using modifiers [4][5][6][7]. Among many modifiers, the styrene butadiene styrene (SBS) is capable of forming a physically cross-linked network structure with asphalt under appropriate conditions. Thus, SBS improves the deformation resistance under high temperatures and cracking resistance at low temperatures, which contributes to its wide application [8]. However, with the continuous increase in pavement materials, and emphasized the improvement effect of the asphalt performance [30,31]. Lv et al. compared the modification effects of five crop bio-oils on rock asphalt and explained the modification mechanism [32].
In summary, previous studies have focused primarily on the modification of asphalt using ash after the combustion of AW and extracting bio-oil through technical treatment. Based on the above investigation, biological ash generally improves the high-temperature performance of asphalt, and the influence of biomass oil prepared by fast pyrolysis, chemical extraction or other methods on asphalt is mainly reflected in the improvement of low-temperature ductility.
However, with global attention to reuse of resources, and the strict requirements for environmental protection, the burning of AW using similar treatment methods is not advocated, particularly in China. Moreover, the treatment of AW combustion is strictly prohibited. In addition, bio-oil's complex purification and treatment technology occupy more resources and energy, and their application potential is limited. In view of the current problems in the application of AW in asphalt modification and the demand for continuous improvement of the SBS asphalt performance caused by extreme high-temperature climatic conditions. In the context of the sustainable use of resources, the application of more convenient agricultural wastes in SBS asphalt needs to be studied.
In this study, AW was directly processed into powder by physical and mechanical processing. Additionally, its effect as a modifier on the high-temperature properties and anti-aging properties of SBS asphalt were explored. The effects of four types of AW on the rheological properties of SBS asphalt under dynamic shear rheometer (DSR) and multiple stress creep recovery test (MSCR) were investigated. Further the potential of AW as an anti-aging agent was evaluated by comparing the rheological test results of SBS asphalt modified by AW before and after aging. Finally, the modification and anti-aging mechanisms of the four AW on SBS asphalt were revealed from the perspective of functional groups through fourier transform infrared spectroscopy (FTIR) analysis.
The innovations of this study are as follows.
• Explored the feasibility of bamboo, rape straw, corn cob, and wheat straw after physical grinding as modifier of SBS asphalt binder.
• Evaluated the improvement of high temperature and aging resistance of four AW.
• Analyzed the microscopic interaction mechanism between AW and SBS asphalt from the perspective of functional groups.

Materials and methods
Herein, SBS asphalt is prepared from 70# base asphalt with SBS modifier. The four types of AW that were considered herein, namely yellow bamboo waste (BP), rape straw (RS), corn cob (CC), and wheat straw (WS) (all in yellow powder form), are shown in Fig 1. The base asphalt and SBS modifier were provided by the China Petroleum & Chemical Corporation, and the AW powder was purchased from Fenglian Company. The physical parameters of base asphalt, SBS and four types of Agricultural Waste are shown in Table 1.

Preparation process of modified asphalt
The preparation method of composite modified asphalt was introduced in related studies [37][38][39][40]. The process of preparing AW/SBS composite modified asphalt binder used was as follows.
1. The base asphalt was heated to 175 ± 5˚C, following which 5% (by weight of base asphalt) SBS modifier was added, under this temperature condition, the asphalt has sufficient

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Study on the high-temperature and aging properties of agricultural waste-modified asphalt based on rheology mobility to better cross-link with the SBS modifier. Next, a high-speed shearing instrument was used for shearing at 175 ± 5˚C for 30 min at a speed of 6000 r/min to prepare SBS asphalt for later use.
2. The SBS asphalt was preheated to 175 ± 5˚C, the selection of material contents refers to the same type of research [41][42][43][44], the different AW powder (5%, 15%, and 15% by weight of SBS asphalt) were added in batches, and a glass rod was used to stir and disperse the powder. Subsequently, the mixture of AW and SBS asphalt was placed in a high-shear machine for high-speed shearing. In order to fully mix AW and SBS asphalt to achieve a better modification effect, the rotation speed and time were set to 6000 r/min and 30 min, respectively.
3. The test asphalt binder samples were numbered to facilitate the analysis and introduction. In this study, the four types of AW modified with SBS asphalt are called AW/SBS asphalt, which does not contradict the name of each asphalt. Table 2 presents the numbering of the different AW/SBS composite modified asphalt binders.
The experiment and test sample information in this study is shown in Table 3.

DSR test
The DSR test was used to measure the complex modulus (G*) and phase angle (δ) of asphalt, which characterize the shear deformation resistance and viscoelastic properties of asphalt. The test results assisted in characterizing the viscoelastic properties of asphalt under the loading condition of cyclic oscillation. In this study, the examination of asphalt followed the test specification AASHTO T315 [45], and the superpave rutting factor (G*/sinδ) was obtained as an index for evaluating the rutting resistance of the asphalt binder. According to the asphalt properties and laboratory conditions, the test temperature range was set to 52-82˚C. In the report of Bahia et al. [46], it was stated that in loading process, the dissipated energy per cycle is inverse to the G*/sinδ represented in Eq (1).
Where U is the energy loss per loading cycle (kPa), G* is the complex modulus (kPa), δ is the phase angle, and σ is the stress level (kPa).

MSCR test
The intermittent loading condition is a unique mode of the MSCR test. MSCR test can obtain two important parameters to evaluate the rheological properties of asphalt, which are irrecoverable creep compliance (J nr ) and elastic recovery rate (R). Their calculations are shown in Eq (2) and Eq (3). The test was performed at two levels of 0.1 kPa and 3.2 kPa, and the test

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Study on the high-temperature and aging properties of agricultural waste-modified asphalt based on rheology temperature was 64˚C. Test operations were performed according to AASHTO T350 [47].
Rð%Þ ¼ Where R (%) is the recovery rate, ε 1 is the strain at the end of 1s creep loading, ε 10 is the strain at the end of the recovery period, J nr is the unrecoverable value (kPa -1 ), and σ is the stress condition (kPa) at each loading stage.

Rotating thin film oven test
In this study, the asphalt is aged with reference to the method of ASTM D2872 [48], and the aged asphalt can be obtained. The change of rheological properties of the aged asphalt can reflect the anti-aging properties. The anti-aging properties of asphalt binders were quantified using complex modulus aging index (CAI) and phase angle aging index (PAI) to evaluate the potential of the four AW as anti-aging agents for SBS asphalt [49,50]. The calculation formula for the aging indices is presented in Table 4.

FTIR test
The change in the macroscopic characteristics of asphalt is reflected at the microscopic level, with the change in the microscopic functional groups as one of the manifestation [51][52][53]. In general, the functional groups of the material correspond to different characteristic peaks in FTIR tests. Therefore, FTIR was used to investigate the chemical structures of the road materials. The spectrogram of asphalt is helpful to qualitatively analyze the modification mechanism of asphalt, while the functional group index can quantitatively analyze the aging mechanism. The test uses the more convenient ATR mode and the wave number used in the test was set to 500-4000 cm -1 , and the spectra, including the calculation of peak areas, were processed using EZ OMNIC software.

DSR test analysis
The results of G* and δ of the AW/SBS composite modified asphalt with varying contents are shown in Fig 3( Fig 3 shows that the addition of four different AW led to a distinct increase in the G* of SBSMA at the same test temperature. This effect is evident with increasing incorporation. The G* values of the four different AW/SBS composite modified asphalts all reached the maximum when the AW content was 15%. The G* value of SBSMA increased, thus indicating improved shear deformation resistance under the action of AW. Moreover, δ reflects the viscoelastic response of asphalt materials [54,55]. Under the same test conditions, the larger the content of BP, RS, and CC, the smaller the δ of the SBSMA, this observation indicates that these three types of AW can significantly enhance the elastic response of the SBSMA. In particular, the elastic response of the SRS is most pronounced. Evidently, WS affected the δ of SBSMA in different ways at different temperatures. Above 64˚C, adding WS to the asphalt increased its δ. However, at 52-64˚C, WS increased the elastic response of the asphalt, and beyond 64˚C, the SBS asphalt was more inclined to a viscous response. Fig 4(a)-4(d) reveal that at the same test temperature, the G*/sinδ values of the four AW/ SBS composite modified asphalts exhibited an increasing trend with increasing AW content. In addition, AW most effectively improved the high-temperature rutting resistance of SBSMA at a content of 15%, thus indicating the ability of the four types of AW to enhance SBSMA in terms of rutting resistance under high temperatures. Note that among the four AW modifiers, RS exhibited the most significant improvement in rutting resistance to the high temperature of SBSMA. This may be because of the interaction between AW and SBS asphalt, which renders the SBS asphalt with superior high-temperature rutting resistance. Evidently, the addition of AW led to a distinct improvement in R, and both the R 0.1 and R 3.2 of SBSMA increased as the AW content increases. When the AW content reached 15%, R 0.1 and R 3.2 of AW/SBS composite modified asphalt exhibited the maximum increase. This increase indicates that the proportion of elastic response deformation to total deformation increased after SBSMA modification under stress, which is consistent with the previous analysis of phase

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Study on the high-temperature and aging properties of agricultural waste-modified asphalt based on rheology angle reduction. It is noteworthy that the R of SBSMA increases more significantly under the stress condition of 3.2 kPa. Different stress levels represent traffic load conditions with different magnitudes. The elastic recovery ability under these conditions was more significant, and the modification effect of RS was the most significant. Both J nr0.1 and J nr3.2 of the SBSMA decreased with increasing content. When the content of AW powder reached 15%, the phases of J nr0.1 and J nr3.2 of AW/SBS composite modified asphalt exhibited the most distinct decrease compared with SBSMA. Evidently, the smaller the J nr value, the smaller the irrecoverable deformation caused by the viscous response of the SBSMA, and the better high-temperature deformation resistance [56]. Both MSCR and DSR were tests for evaluating high-temperature properties of asphalt. It was interesting to note that AW affects G*/sinδ, and R of SBS asphalt in a similar trend, presumably because the physical properties of the four types of AW are very close to each other.

Analysis of aging indices
The original data for calculating the aging indices were all calculated at 64˚C to facilitate the correlation analysis between the microscopic and the macroscopic rheological indices below.

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Study on the high-temperature and aging properties of agricultural waste-modified asphalt based on rheology 1. The complex modulus aging index CAI exhibited a decreasing trend with increasing AW content. The CAI value of AW/SBS modified asphalt with a content of 15% was the smallest, and the smaller the CAI value, the higher the AW/SBS content.
2. The phase angle aging index PAI constantly increased as the content increased. The PAI value of the AW/SBS modified asphalt with a content of 15% was the largest. A larger PAI value indicates improved anti-aging properties.
3. The analysis of the CAI and PAI aging indices revealed that the four types of AW remarkably improved the anti-aging properties of SBSMA. Among them, SRS modified asphalt exhibited the most robust anti-aging properties.

FTIR test analysis
The four types of AW remarkably enhanced the high-temperature deformation resistance and anti-aging properties of SBSMA. Consequently, we became interested in exploring the microscopic mechanism of the action of AW on SBSMA. Thus, the modification and anti-aging mechanisms of the four types of AW and SBSMA were explored. The absorption peaks at 2921 cm -1 and 2851 cm -1 corresponded to the characteristic peaks of aliphatic C-H stretching vibration, that at 1599 cm -1 corresponded to the stretching vibration of aromatic C-C, those at 1456 cm -1 and 1376 cm -1 were the bending vibration absorption peak of C-H, and the stretching vibration of sulfoxide group (S = O) at 1030 cm -1 , respectively. The absorption peak at 700 cm -1 -862 cm -1 was the characteristic peak of the C-H bending vibration on the benzene ring. In addition, characteristic peaks of the polymer were observed at 699 cm -1 and 966 cm -1 , of which 699 cm -1 corresponded to the vibration absorption peak of the mono-substituted benzene ring of polystyrene (PS) in SBS, whereas that at 966 cm -1 was the polybutylene twisting vibrational absorption peak of C = C bonds in polybutadiene (PB).
If the chemical reaction occurs in the modification process of asphalt, the chemical structure of the molecule will change under the strong interaction, and the infrared spectrum of asphalt after the modification will also change compared with the original asphalt spectrum. Consequently, the appearance of new characteristic absorption peaks is the most common phenomenon. A comparison of the infrared spectra of SBSMA and SBP/SBS, SWC/SBS, SCC/ SBS, and SRS/SBS composite modified asphalt binders revealed that the peak phases of the characteristic absorption peaks were close to each other. The AW powder is dispersed and mixed inside the SBSMA to increase the viscosity of the SBSMA. The close physical properties of the four AW powder, such as particle size and density, indicates that the addition of AW did not alter the molecular properties of SBSMA. Hence, the spectra of the modified asphalt binders are very similar to the original SBSMA binder [57][58][59]. In addition, no new characteristic absorption peak appeared. Thus, the modification process between the four types of AW and SBSMA can be regarded as a physical modification.
To further explore the influence of AW on the microchemical structure of SBSMA aging behavior, the characteristic peak area parameters of the sulfoxide group (S = O) were selected for calculation and analysis. The sulfoxide index (SI) represents the change in sulfoxide

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Study on the high-temperature and aging properties of agricultural waste-modified asphalt based on rheology functional groups and has been widely used in asphalt aging research [60]. In this study, SI was used as the quantitative analysis index to explore anti-aging properties of SBS asphalt. It was calculated as using the Eq (4), as follows.
where A S = O is the absorption peak area centered at 1030 cm -1 , and A is the absorption peak area centered at 1456 cm -1 . Under the action of aging conditions, the polar groups in the asphalt that are sensitive to high temperature and oxygen conditions changed. The sulfide in the asphalt participated in the oxidation reaction, thereby causing the content of sulfoxide groups, located at 1030 cm -1 , to change. Consequently, the characteristic peak area was affected. R-SBS and aged AW/SBS exhibited larger characteristic peak areas of sulfoxide groups at 1030 cm -1 compared with unaged asphalt. I B/S is a reference index used to characterize the polymer content in the SBS asphalt. It is the ratio of the peak area centered at 966 cm -1 to that centered at 699 cm -1 . SBSMA under aging conditions, the 966 cm -1 peak changes significantly, while the 699 cm -1 peak is more stable, so this ratio can reflect the polymer content. Detailed studies on asphalt aging have been conducted [61]. The calculation of I B/S is shown in Eq (5), as follows.
where A 966 cm -1 is the absorption peak area centered at 966 cm -1 , and A 699 cm -1 refers to the area centered at 699 cm -1 . Compared with SBSMA, R-SBS exhibited an enhanced absorption peak of the sulfoxide group (S = O) at 1030 cm -1 , and the peak area increased.
The SI and I B/S results of SBSMA, and the four AW/SBS composite modified asphalts and their aged samples are shown in Fig 8. First, the SI analysis of the asphalt system revealed no distinct difference between the SI values of the four AW/SBS composite modified asphalts and the SI values of SBSMA. This is due to the changes in the sulfoxide group are caused by the oxidation reactions, while the change in polymer content is only related to the polymer additive content of SBS asphalt itself. The addition of the AW did not affect their chemical bonds. After RTFOT, the SI of the SBSMA and four AW/SBS modified asphalts increased. In contrast, the AW/SBS modified asphalt exhibited a smaller increase in SI relative to SBSMA, and the larger the AW content, the smaller the increase in SI. The increases in SI of SBP15, SWS15, SCC15, SRS15, and SBSMA were 61.2%, 60.2%, 61.7%, 56.6%, and 129.79%, respectively. The SI increases for the four types of AW/SBS composite modified asphalt with the content in the AW content reaching 15% were much smaller than those of SBSMA, which indicates that the four types of AW weakened the sensitivity of the S = O to the thermal-oxygen environment and inhibited the growth of the S = O.
Second, from the perspective of polymer analysis of I B/S , the SBS modifier had the same content in the four AW/SBS composite modified asphalt and SBSMA. After aging, I B/S values decreased for all five asphalt binders. The decreases in I B/S of SBP15, SWS15, SCC15, SRS15, and SBSMA were 17.19%, 16.95%, 15.9%, 14.96%, and 35.88%, respectively. The I B/S values of AW/SBS composite modified asphalt with an AW content of 15% exhibited a smaller decrease compared with the SBSMA, thus indicating that the addition of AW can effectively inhibit the cracking behavior of polymer under aging conditions. When SBSMA was aged, AW relieved the growth of asphalt sulfoxide groups and the cracking of polymer under the aging effect of SBSMA through the mutual adsorption of sensitive segments in asphalt and polymer, thereby improving the anti-aging properties.

Correlation analysis
Previous studies have shown that the properties of asphalt is affected when it is affected by modifiers or aging conditions. This is because the sensitive functional groups in asphalt are highly sensitive to modifiers or aging conditions, these sensitive functional groups change under external factors, thereby affecting the asphalt performance. In the previous FTIR analysis, SI was selected as the reference index to explore microscopic mechanism of AW/SBS composite modified asphalt. This section introduces a new index ΔSI, whose calculation method is shown in Eq (6). Table 5 lists the sulfoxide group index SI and high-temperature rheological indices of SBSMA and AW/SBS composite modified asphalt (G*, δ, G*/sinδ, R 0.1 , R 3.2 , J 0.1 , and J 3.2 ). Fig 9 shows the association of the high-temperature rheological indices with the SI. Among the high-temperature rheological indices, the correlation between δ and SI was the best (R 2 =

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0.8003), which suggests that the phase angle can not only serve as an index for inversely evaluating the elastic response of asphalt materials but also can be used as a reference to reflect the SI content of AW/SBS composite modified asphalt. Table 6 lists the ΔSI, R-I B/S and aging indices (CAI and PAI) of the SBSMA and AW/SBS composite modified asphalt (in the analysis of this section, the 15% content of AW/SBS modified asphalt with the most significant modification effect was selected).
Relevant studies have highlighted that SBSMA has a complex aging process, and two processes of polymer cracking and base asphalt hardening occur simultaneously. In the FTIR analysis part of this study, SI and R-I B/S were used to analyze the mechanism of aging reaction. The relationship between polymer cracking reactions during aging of AW/SBS modified asphalt and high-temperature rheological properties of SBSMA before and after aging, as well as the relationship between the hardening reactions of asphalt systems and high-temperature rheological properties of SBSMA before and after aging, respectively, deserve further investigation. The aging indices (CAI and PAI) obtained based on high-temperature rheological indices were compared with ΔSI to explore the association of the aging indices with the sulfoxide group in the asphalt system, the aging indices (CAI and PAI) and R-I B/S obtained from the high- temperature rheological indices were used to analyze the association of the aging indices with polymer cracking. The relationship between aging indices and R-I B/S is presented Fig 10(A). The correlation between the PAI (R 2 = 0.9988) and R-I B/S was the best. During the aging process, SBS modifier degradation is an important factor that affects the SBSMA performance. The larger the R-I B/S , the higher the SBS modifier content after aging, and the lower the cracking degree.
The relationship between aging indices and ΔSI is illustrated in Fig 10(B). The best correlation between the CAI (R 2 = 0.9953) and ΔSI represents the change in the G* before and after aging. Smaller CAI values indicate that the asphalt has better aging resistance, and therefore the shear deformation resistance is less affected by the aging test. ΔSI shows the change in the sulfoxide group content before and after asphalt aging, which corresponds to the analysis of the sulfoxide group content above, thus indicating that CAI can be used to reflect the change in sulfoxide group content in the aging reaction of the AW/SBS composite-modified asphalt.

Conclusion
In this study, four types of AW, namely BP, WS, CC, and RS, were used to modify SBSMA. The effect of different contents of AW on the rheological properties of SBSMA was evaluated by DSR and MSCR tests. The modified asphalt binder was subjected to thermo-oxidative aging to explore its aging characteristics. Finally, the microscopic action mechanism of AW on SBSMA was analyzed using a microscopic FTIR test from the perspective of functional groups. The study following four main conclusions were drawn.

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1. The analysis of DSR test results revealed that the four types of AW contributed to an obvious improvement in the high-temperature properties of SBSMA, and the effect was significant as the content of AW increased.
2. The R value and J nr value under different stress in the MSCR test revealed that the four types of AW could enhance the rutting resistance of SBSMA, and improvement effect was more evident under the condition of heavy traffic. Among them, the modification of SBSMA by RS was the most effective.
3. The test results of RTFOT revealed that adding AW improved the anti-aging properties of SBSMA, thus indicating that the four types of AW have the potential to be used as antiaging agents for SBSMA. Similarly, the modification of SBSMA by RS was the most significant.
4. The qualitative analysis of the FTIR spectrum revealed that the modification process of the four types of AW and SBSMA was performed in the form of physical blending. Furthermore, quantitative analysis of the functional group index revealed that AW could simultaneously inhibit the growth of SI in SBSMA under thermo-oxidative aging conditions and inhibit the cracking of the SBS modifier, thereby enhancing the anti-aging ability of SBSMA.
In this study, AW/SBS modified asphalts have better high-temperature properties and antiaging properties compared with SBSMA, which provides a reference for the construction of agricultural waste asphalt pavement in perennial high-temperature areas.

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Study on the high-temperature and aging properties of agricultural waste-modified asphalt based on rheology